Resolvers are sometimes used as a rotation detection sensor that detects rotation (the rotational speed or rotational position (angle of rotation) of a rotor) of a rotating electrical machine. The resolvers typically have an excitation winding on the stator side, and detect rotation of the rotating electrical machine based on a principle that the angle of rotation of a rotor is detected according to an output voltage induced to an output winding provided on the rotor side. Accordingly, an alternating current (AC) signal is normally applied to the excitation winding. An excitation apparatus that applies this AC signal has a sinusoidal oscillator circuit that generates a sinusoidal oscillation signal, and a driver circuit (e.g., amplifier circuit) that increases the amplitude of the sinusoidal oscillation signal to supply the resultant sinusoidal oscillation signal to the excitation winding. Japanese Patent Application Publication No. 2009-180585 (JP 2009-180585 A) (Patent Document 1) discloses an example, using as the driver circuit a series resonant circuit having a series resonant capacitor for the excitation winding of the resolver (Paragraph [0006], FIG. 5, etc.). According to Patent Document 1, capacitance of this capacitor and inductance of the excitation winding are set so as to increase the quality factor Q of the resonant circuit, and the amplitude of an excitation signal is adjusted by appropriately setting the frequency of the excitation signal. That is, the amplitude of the sinusoidal oscillation signal generated by the sinusoidal oscillator circuit is increased and applied to the excitation winding.
The excitation apparatus with such a configuration has a smaller circuit size and contributes to reduction in size and cost, as compared to the case of excitation apparatuses having an amplifier circuit using an operational amplifier etc. However, applying such a circuit requires conditions to be satisfied such as that the excitation winding has a sufficiently small direct current (DC) resistance component (i.e., the quality factor Q is high), and that the amplitude of the signal that is applied to the excitation winding is about the same as that of a power supply voltage of typical electronic circuits (e.g., about 5 [V] as shown in Patent Document 1) and the increase rate of the amplitude is relatively low.
On the other hand, in the resolvers that detect rotation of the rotating electrical machine used as a driving force source or a power source of hybrid cars, electric cars, etc. that have been increasingly demanded in recent years, the AC signal that is applied to the excitation winding is required to have a peak value (peak-to-peak) of about 20 to 30 [V]. Since a source signal of the excitation signal is generated by the typical electronic circuits, its peak value is about 2 to 3 [V], and a relatively high increase rate is required. The circuits that generate the source signal are often used for general purposes, and it is possible to select a single frequency for the excitation signal, but the frequency of the excitation signal is normally fixed to 10 [kHz], 20 [kHz], etc. in many cases. Accordingly, it is not practically easy to flexibly and appropriately set the frequency of the excitation signal as in Patent Document 1. The excitation apparatus that excites such a resolver therefore includes an amplifier circuit having an amplification factor of 10 in many cases.
A power supply having a relatively large power supply voltage range (about 25 to 35 [V]) is required in order that an excitation signal having a peak value of about 20 to 30 [V] is obtained. However, loss of the circuit tends to increase with an increase in power supply voltage. Accordingly, there are cases where the loss of the circuit (excitation apparatus) is reduced by adding a parallel resonant capacitor for the excitation winding to form a parallel resonant circuit. The loss is small if the excitation apparatus includes a power supply having a power supply voltage value optimal for the excitation apparatus, but otherwise the loss may increase. For example, if the voltage value of the power supply included in the excitation apparatus is higher than the optimal power supply voltage value, the loss is greater as the power supply voltage value is higher. If the voltage value of the power supply included in the excitation apparatus is lower than the optimal power supply voltage value, the excitation apparatus cannot provide a sinusoidal output having required amplitude, and provides an output having a waveform distorted with respect to the sinusoidal waveform.